I was told that epsom salts work as a mild laxative, 1 cup/100g. I don't know the size of your tank, but if it's smaller than 100 gallons, divide the portion accordingly. Be sure to dissolve the epsom salt in water before adding it to the tank.
Hope this helps you out
If he's eating, try feeding Peas, or use a garlic product with his food and it will hopefully make things easier for him. New Life Spectrum food contains garlic so I would highly recommend that.
Live feeders may be a big part of your problem. As has been stated countless other times on this forum, feeders hold little nutritional value, and often carry many parasites and diseases which can be detrimental to the health of your fish.
Try a high quality pellet or any number of dry or frozen foods available.